arXiv subjects

Mireille Aymar

Publications and source records attributed to Mireille Aymar.

16 recordsLinked to original sources

arXiv2015

Formation of molecular ions by radiative association of cold trapped atoms and ions

Radiative emission during cold collisions between trapped laser-cooled Rb atoms and alkaline-earth ions (Ca$^+$, Sr$^+$, Ba$^+$) and Yb$^+$, and between Li and Yb$^+$, are studied theoretically, using accurate effective-core-potential based quantum chemistry calculations of potential energy curves and transition dipole moments of the related molecular ions. Radiative association of molecular ions is predicted to occur for all systems with a cross section two to ten times larger than the radiative charge transfer one. Partial and total rate constants are also calculated and compared to available experiments. Narrow shape resonances are expected, which could be detectable at low temperature with an experimental resolution at the limit of the present standards. Vibrational distributions are also calculated, showing that the final molecular ions are not created in their ground state level.

physics.atom-ph↗

arXiv2013

Long-range interactions between polar alkali-metal diatoms in external electric fields

We computed the long-range interactions between two identical polar bialkali molecules in their rovibronic ground level, for all ten species involving Li, Na, K, Rb and Cs, using accurate quantum chemistry results combined with available spectroscopic data. Huge van der Waals interaction is found for eight species in free space. The competition of the van der Waals interaction with the dipole-dipole interaction induced by an external electric field parallel or perpendicular to the intermolecular axis is investigated by varying the electric field magnitude and the intermolecular distance. Our calculations predict a regime with the mutual orientation of the two molecules but with no preferential direction in the laboratory frame. A mechanism for the stimulated one-photon radiative association of a pair of ultracold polar molecules into ultracold tetramers is proposed, which would open the way towards the optical manipulation of ultracold polyatomic molecules.

physics.atom-ph↗

arXiv2013

Ion-neutral chemistry at ultralow energies: Dynamics of reactive collisions between laser-cooled Ca^+ ions and Rb atoms in an ion-atom hybrid trap

Cold chemical reactions between laser-cooled Ca^+ ions and Rb atoms were studied in an ion-atom hybrid trap. Reaction rate constants were determined in the range of collision energies /k_B = 20 mK-20 K. The lowest energies were achieved in experiments using single localized Ca^+ ions. Product branching ratios were studied using resonant-excitation mass spectrometry. The dynamics of the reactive processes in this system (non-radiative and radiative charge transfer as well as radiative association leading to the formation of CaRb^+ molecular ions) have been analyzed using high-level quantum-chemical calculations of the potential energy curves of CaRb^+ and quantum-scattering calculations for the radiative channels. For the present low-energy scattering experiments, it is shown that the energy dependence of the reaction rate constants is governed by long-range interactions in line with the classical Langevin model, but their magnitude is determined by short-range non-adiabatic and radiative couplings which only weakly depend on the asymptotic energy. The quantum character of the collisions is predicted to manifest itself in the occurrence of narrow shape resonances at well-defined collision energies. The present results highlight both universal and system-specific phenomena in cold ion-neutral reactive collisions.

physics.atom-ph↗

arXiv2013

Light-Assisted Cold Chemical Reactions of Barium Ions with Rubidium Atoms

Light-assisted reactive collisions between laser-cooled Ba+ ions and Rb atoms were studied in an ion-atom hybrid trap. The reaction rate was found to strongly depend on the electronic state of the reaction partners with the largest rate constant (7(2) x 10^-11 cm^3 s^-1) obtained for the excited Ba+(6s)+Rb(5p) reaction channel. Similar to the previously studied Ca+ + Rb system, charge transfer and radiative association were found to be the dominant reactive processes. The generation of molecular ions by radiative association could directly be observed by their sympathetic cooling into a Coulomb crystal. Potential energy curves up to the Ba+(6s)+Rb(5p) asymptote and reactive-scattering cross sections for the radiative processes were calculated. The theoretical rate constant obtained for the lowest reaction channel Ba+(6s)+Rb(5s) is compatible with the experimental estimates obtained thus far.

physics.atom-ph↗

arXiv2011

Light-assisted ion-neutral reactive processes in the cold regime: radiative molecule formation vs. charge exchange

We present a combined experimental and theoretical study of cold reactive collisions between laser-cooled Ca+ ions and Rb atoms in an ion-atom hybrid trap. We observe rich chemical dynamics which are interpreted in terms of non-adiabatic and radiative charge exchange as well as radiative molecule formation using high-level electronic structure calculations. We study the role of light-assisted processes and show that the efficiency of the dominant chemical pathways is considerably enhanced in excited reaction channels. Our results illustrate the importance of radiative and non-radiative processes for the cold chemistry occurring in ion-atom hybrid traps.

physics.atom-ph↗

arXiv2011

Structure of the Alkali-metal-atom-Strontium molecular ions: towards photoassociation and formation of cold molecular ions

The potential energy curves, permanent and transition dipole moments, and the static dipolar polarizability, of molecular ions composed of one alkali-metal atom and a Strontium ion are determined with a quantum chemistry approach. The molecular ions are treated as effective two-electron systems and are treated using effective core potentials including core polarization, large gaussian basis sets, and full configuration interaction. In the perspective of upcoming experiments aiming at merging cold atom and cold ion traps, possible paths for radiative charge exchange, photoassociation of a cold Lithium or Rubidium atom and a Strontium ion are discussed, as well as the formation of stable molecular ions.

quant-ph↗

arXiv2011

Optimal trapping wavelengths of Cs$_2$ molecules in an optical lattice

The present paper aims at finding optimal parameters for trapping of Cs$_2$ molecules in optical lattices, with the perspective of creating a quantum degenerate gas of ground-state molecules. We have calculated dynamic polarizabilities of Cs$_2$ molecules subject to an oscillating electric field, using accurate potential curves and electronic transition dipole moments. We show that for some particular wavelengths of the optical lattice, called "magic wavelengths", the polarizability of the ground-state molecules is equal to the one of a Feshbach molecule. As the creation of the sample of ground-state molecules relies on an adiabatic population transfer from weakly-bound molecules created on a Feshbach resonance, such a coincidence ensures that both the initial and final states are favorably trapped by the lattice light, allowing optimized transfer in agreement with the experimental observation.

quant-ph↗

arXiv2010

Triplet-singlet conversion in ultracold Cs$_2$ and production of ground state molecules

We propose a process to convert ultracold metastable Cs$_2$ molecules in their lowest triplet state into (singlet) ground state molecules in their lowest vibrational levels. Molecules are first pumped into an excited triplet state, and the triplet-singlet conversion is facilitated by a two-step spontaneous decay through the coupled $A^{1}Σ_{u}^{+} \sim b ^{3}Π_{u}$ states. Using spectroscopic data and accurate quantum chemistry calculations for Cs$_2$ potential curves and transition dipole moments, we show that this process has a high rate and competes favorably with the single-photon decay back to the lowest triplet state. In addition, we demonstrate that this conversion process represents a loss channel for vibrational cooling of metastable triplet molecules, preventing an efficient optical pumping cycle down to low vibrational levels.

quant-ph↗

arXiv2010

The ground state of the polar alkali-Strontium molecules: potential energy curve and permanent dipole moment

In this study, we investigate the structure of the polar alkali-Strontium diatomic molecules as possible candidates for the realization of samples of new species of ultracold polar molecules. Using a quantum chemistry approach based on Effective Core Potentials and Core Polarization Potentials, we model these systems as effective three valence electron systems, allowing for calculation of electronic properties with Full Configuration Interaction. The potential curve and the permanent dipole moment of the $^2Σ^+$ ground state are determined as functions of the internuclear distances for LiSr, NaSr, KSr, RbSr, and CsSr molecules. These molecules are found to exhibit a significant permanent dipole moment, though smaller than those of the alkali-Rb molecules.

physics.atm-clus↗

arXiv2010

Photoionization spectroscopy of excited states of cold cesium dimers

Photoionization spectroscopy of cold cesium dimers obtained by photoassociation of cold atoms in a magneto-optical trap is reported here. In particular, we report on the observation and on the spectroscopic analysis of all the excited states that have actually been used for efficient detection of cold molecules stabilized in the triplet a^3Sigma_u^+ ground state. They are: the (1)^3Sigma_g^+ state connected to the 6s+6p asymptote, the (2)^3Sigma_g^+ and (2)^3Pi_g states connected to the 6s+5d asymptote and finally the (3)^3Sigma_g^+ state connected to the 6s + 7s asymptote. The detection through these states spans a wide range of laser energies, from 8000 to 16500 cm-1, obtained with different laser dyes and techniques. Information on the initial distribution of cold molecules among the different vibrational levels of the a^3Sigma_u^+ ground state is also provided. This spectroscopic knowledge is important when conceiving schemes for quantum manipulation, population transfer and optical detection of cold cesium molecules.

physics.chem-ph↗

arXiv2009

Core repulsion effects in alkali trimers

The present paper is related to a talk presented during the Symposium on Coherent Control and Ultracold Chemistry held during the Sixth Congress of the International Society for Theoretical Chemical Physics (ISTCP-VI, July 2008). The talk was entitled "Electronic structure properties of alkali dimers and trimers. Prospects for alignment of ultracold molecules". Here we report on the electrostatic repulsion forces of the ionic cores at short separation, involved when the potential energy surfaces of alkali trimers are calculated with a quantum chemistry approach based on effective large-core potentials for ionic core description. We demonstrate that such forces in the triatomic molecule can be obtained as the sum of three pairwise terms. We illustrate our results on the lowest electronic states of Cs$_3$, which are computed for the first time within a full configuration interaction based on a large Gaussian basis set. As a preliminary section, we also propose a brief introduction about the importance of alkali trimer systems in the context of cold and ultracold molecules.

physics.atm-clus↗

arXiv2009

Electronic structure of the Magnesium hydride molecular ion

In this paper, using a standard quantum chemistry approach based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective core polarization potentials, we investigate the electronic properties of the MgH$^+$ ion. We first determine potential energy curves for several states using different basis sets and discuss their predicted accuracy by comparing our values of the well depths and position with other available results. We then calculate permanent and transition dipole moments for several transitions. Finally for the first time, we calculate the static dipole polarizability of MgH$^+$ as function of the interatomic distance. This study represents the first step towards the modeling of collisions between trapped cold Mg$^+$ ions and H$_2$ molecules.

quant-ph↗

arXiv2009

Systematic trends in electronic properties of alkali hydrides

Obtaining ultracold samples of dipolar molecules is a current challenge which requires an accurate knowledge of their electronic properties to guide the ongoing experiments. Alkali hydride molecules have permanent dipole significantly larger than those of mixed alkali species and, as pointed out by Taylor-Juarros et al. [Eur. Phys. J. D 31, 213 (2004)] and by Juarros et al. [Phys. Rev. A 73, 041403 (2006)], are thus good candidates for molecule formation. In this paper, using a standard quantum chemistry approach based on pseudopotentials for atomic core representation, large Gaussian basis sets, and effective core polarization potential, we systematically investigate the electronic properties of the alkali hydrides LiH to CsH, in order to discuss general trends of their behavior. We computed (for the first time for NaH, KH, RbH, and CsH) the variation of their static polarizability with the internuclear distance. Moreover, in addition to potential curves, we determine accurate values of permanent and transition dipole moments for ground and excited states depending on the internuclear distance. The behavior of electronic properties of all alkali hydrides is compared to each other, in the light of the numerous other data available in the literature. Finally, the influence of the quality of the representation of the hydrogen electronic affinity in the approach on the results is discussed.

physics.atom-ph↗

arXiv2008

Calculations of static dipole polarizabilities of alkali dimers. Prospects for alignment of ultracold molecules

The rapid development of experimental techniques to produce ultracold alkali molecules opens the ways to manipulate them and to control their dynamics using external electric fields. A prerequisite quantity for such studies is the knowledge of their static dipole polarizabilities. In this paper, we computed the variations with internuclear distance and with vibrational index of the static dipole polarizability components of all homonuclear alkali dimers including Fr$_2$, and of all heteronuclear alkali dimers involving Li to Cs, in their electronic ground state and in their lowest triplet state. We use the same quantum chemistry approach than in our work on dipole moments (M. Aymar and O. Dulieu, J. Chem. Phys. 122, 204302 (2005)), based on pseudopotentials for atomic core representation, Gaussian basis sets, and effective potentials for core polarization. Polarizabilities are extracted from electronic energies using the finite-field method. For the heaviest species Rb$_2$, Cs$_2$ and Fr$_2$ and for all heteronuclear alkali dimers, such results are presented for the first time. The accuracy of our results on atomic and molecular static dipole polarizabilities is discussed by comparing our values with the few available experimental data and elaborate calculations. We found that for all alkali pairs, the parallel and perpendicular components of the ground state polarizabilities at the equilibrium distance $R_e$ scale as $(R_e)^3$, which can be related to a simple electrostatic model of an ellipsoidal charge distribution. Prospects for possible alignment and orientation effects with these molecules in forthcoming experiments are discussed.

physics.atom-ph↗

arXiv2006

Electronic properties of Francium diatomic compounds and prospects for cold molecule formation

In this work we investigate the possibility to create cold Fr$\_2$, RbFr, and CsFr molecules through photoassociation of cold atoms. Potential curves, permanent and transition dipole moments for the Francium dimer and for the RbFr and RbCs molecules are determined for the first time. The Francium atom is modelled as a one valence electron moving in the field of the Fr$^+$ core, which is described by a new pseudopotential with averaged relativistic effects, and including effective core polarization potential. The molecular calculations are performed for both the ionic species Fr$\_2^+$, RbFr$^+$, CsFr$^+$ and the corresponding neutral, through the CIPSI quantum chemistry package where we used new extended gaussian basis sets for Rb, Cs, and Fr atoms. As no experimental data is available, we discuss our results by comparison with the Rb$\_2$, Cs$\_2$, and RbCs systems. The dipole moment of CsFr reveals an electron transfer yielding a Cs$^+$Fr$^-$ arrangement, while in all other mixed alkali pairs the electron is transferred towards the lighter species. Finally the perturbative treatment of the spin-orbit coupling at large distances predicts that in contrast with Rb$\_2$ and Cs$\_2$, no double-well excited potential should be present in Fr$\_2$, probably preventing an efficient formation of cold dimers via photoassociation of cold Francium atoms.

quant-ph↗

arXiv2005

Calculation of accurate permanent dipole moments of the lowest $^{1,3} Σ^+$ states of heteronuclear alkali dimers using extended basis sets

The obtention of ultracold samples of dipolar molecules is a current challenge which requires an accurate knowledge of their electronic properties to guide the ongoing experiments. In this paper, we systematically investigate the ground state and the lowest triplet state of mixed alkali dimers (involving Li, Na, K, Rb, Cs) using a standard quantum chemistry approach based on pseudopotentials for atomic core representation, gaussian basis sets, and effective terms for core polarization effects. We emphasize on the convergence of the results for permanent dipole moments regarding the size of the gaussian basis set, and we discuss their predicted accuracy by comparing to other theoretical calculations or available experimental values. We also revisit the difficulty to compare computed potential curves among published papers, due to the differences in the modelization of core-core interaction.

quant-ph↗